In the marine industry, it is known to use a "whistler" to avoid the overfilling of a fuel tank. A whistler placed in the overflow line of a fuel system whistles during refueling until it becomes submerged in fuel flowing into the overflow line. A problem with this mechanism is that, by the time the whistler becomes wet, it is too late to prevent the spillage of fuel from the overflow line into the surrounding body of water. Such spillage of fuel into the surrounding water is harmful to the environment.
It is also known to use a sight glass in an overflow line. In theory, one should be able to detect an impending overflow by monitoring the sight glass. However, it is common for multiple overflow lines to be placed on opposite sides of the vessel, so that fuel can overflow from an overflow line that is not within the operator's field of view. In addition, it is not uncommon for the overflow lines to be unlabeled, which can lead to fuel spillage. A goal of the present invention is to prevent such spillage.